From what I've seen it appears that areas get a particular level of assailant. And that over time, if an assailant isn't killed off, it disappears (same with Assailant's Camps that don'e get emptied). Then new assailants (often of a different type such as Rebel, Disciple, Fugitive, etc) of the same level then repopulate the area. But they definetly do not seem to grow in level over time.